19Q9 — Medium-Mu Triode and Semi-Remote Cutoff Pentode

The 19Q9 is a multi-section miniature vacuum tube containing a medium-mu triode and a semi-remote cutoff pentode within a single envelope. It is designed primarily for use in FM radio receivers as an RF amplifier and autodyne mixer. The tube features a 10-pin miniature construction and is designed for reliable operation in compact high-frequency circuits.

Physical Construction and Basing

The 19Q9 is housed in a T-6 1/2 glass bulb with an outline designation of 6-13. It utilizes an E10-73 10-pin button base, which is structurally based on the standard E9-1 9-pin miniature base but with an additional center pin. This configuration requires a specialized miniature 10-contact socket.

The tube can be mounted in any position. Its physical dimensions are as follows:

• Maximum Diameter: 0.875 inches
• Maximum Seated Height: 1.940 inches
• Maximum Overall Height: 2.190 inches

Pinout and Electrode Connections

The 19Q9 uses the 10H basing diagram. The pin connections, viewed from the bottom of the tube, are assigned as follows:

The cathodes of both the triode and pentode sections are coated, unipotential designs. The separate cathode connections (Pin 7 for the triode and the Center Pin for the pentode) allow the two sections to be biased independently.

Heater Characteristics

The heater of the 19Q9 is designed for series-string or parallel operation, featuring a controlled warm-up time to minimize voltage surges in series-connected receiver strings.

Heater-Cathode Voltage Ratings

To prevent insulation breakdown between the heater and the separate cathodes, designers must adhere to the following design-maximum limits:

• Heater Negative with Respect to Cathode: 200 V Max (Total DC and Peak)
• Heater Positive with Respect to Cathode:
  • DC Component: 100 V Max
  • Total DC and Peak: 200 V Max

Electrical Characteristics and Typical Operation

The 19Q9 operates as a Class A1 amplifier. The triode and pentode sections exhibit distinct electrical properties tailored to their respective roles in RF amplification and mixing.

Triode Section

The triode section is characterized by a medium amplification factor and relatively high transconductance, making it suitable for RF voltage amplification.

• Plate Voltage: 125 V
• Grid No. 1 Voltage: -1.0 V
• Plate Current: 14 mA
• Transconductance: 8000 µmhos
• Amplification Factor (μ): 40
• Plate Resistance (Approx.): 5000 Ω
• Grid No. 1 Voltage for Cutoff (at Plate Current = 20 µA): -9 V (Approx.)

Pentode Section

The pentode section features a semi-remote cutoff characteristic, allowing for effective automatic gain control (AGC) action when used as an RF amplifier. It can be operated under different voltage conditions:

Design-Maximum Ratings and Circuit Constraints

The ratings below are established under the Design-Maximum Rating System. Operating the tube beyond these limits may impair its performance and shorten its operating life.

Grid Circuit Resistance Limits

To prevent thermal runaway and maintain stable bias, the maximum resistance in the control grid circuit must not exceed the following values:

• Triode Section:
  • Fixed Bias: 0.5 MΩ Max
  • Self (Cathode) Bias: 1.0 MΩ Max
• Pentode Section:
  • Fixed Bias: 0.25 MΩ Max
  • Self (Cathode) Bias: 1.0 MΩ Max

Triode Testing Caution

The physical construction of the triode section features an extremely close spacing between the control grid and the cathode to achieve its high transconductance. Consequently, users must exercise caution during testing:

• Voltage Limit: Do not exceed 100 V DC or peak AC between the triode control grid and cathode in commercial tube checkers or shorts-indicating devices.
• Mechanical Excitation: This voltage limit is especially critical when the tube is subjected to mechanical vibration or tapping during testing, as physical contact between the closely spaced elements could cause permanent damage or false short-circuit indications.

Direct Interelectrode Capacitances

Because the 19Q9 is intended for high-frequency RF applications, its internal capacitances are critical design parameters. The values below compare the capacitances with and without an external shield (Shield No. 315).

Triode Section Capacitances

• Grid to Plate: 1.7 pF (Shielded or Unshielded)
• Input [g to (h + k + Pk + IS)]: 3.2 pF (Shielded or Unshielded)
• Output [p to (h + k + Pk + IS)]: 1.9 pF (Shielded) / 1.1 pF (Unshielded)
• Heater to Cathode: 3.0 pF (Shielded or Unshielded)

Pentode Section Capacitances

• Grid No. 1 to Plate: 0.035 pF Max (Shielded) / 0.050 pF Max (Unshielded)
• Input [g1 to (h + k + g2 + Pk + IS)]: 5.0 pF (Shielded or Unshielded)
• Output [p to (h + k + g2 + Pk + IS)]: 3.3 pF (Shielded) / 2.4 pF (Unshielded)
• Heater to Cathode: 3.0 pF (Shielded or Unshielded)

Inter-Section Coupling Capacitances

To minimize unwanted feedback and crosstalk between the triode and pentode sections, the coupling capacitances are kept extremely low, particularly when an external shield is employed:

• Pentode Grid No. 1 to Triode Plate: 0.030 pF Max (Shielded) / 0.035 pF Max (Unshielded)
• Pentode Plate to Triode Plate: 0.030 pF Max (Shielded) / 0.13 pF Max (Unshielded)
• Triode Grid to Pentode Plate: 0.002 pF Max (Shielded) / 0.009 pF Max (Unshielded)

Note on Shielding: When measuring shielded capacitances, Shield No. 315 is connected to Pin 4 or Pin 3 as specified in standard testing procedures.

Technical Summary

The 19Q9 is a specialized, high-performance multi-section tube combining a medium-mu triode and a semi-remote cutoff pentode in a compact T-6 1/2 10-pin miniature envelope. Operating with a 18.9 V, 150 mA heater, it is optimized for series-string FM receivers.

The triode section provides a transconductance of 8000 µmhos and an amplification factor of 40 at a typical plate voltage of 125 V, while the pentode section offers up to 7000 µmhos of transconductance. With design-maximum plate ratings of 330 V and carefully controlled interelectrode capacitances, the 19Q9 serves as an efficient and stable solution for FM RF amplification and autodyne mixer applications. Due to the extremely close grid-to-cathode spacing in the triode section, care must be taken during testing not to exceed 100 V between these elements.